Low oxygen content fine spherical copper particles and process for producing same by fluid energy milling and high temperature processing

ABSTRACT

A powder material and a process for producing same are disclosed. The process comprises reducing the size of an electrolytically produced starting dendritic copper powder material by fluid energy milling to produce a finer powder, essentially all of which has a particle size of less than about 20 micrometers in diameter, entraining the finer powder in a carrier gas and passing it through a high temperature zone at a temperature above the melting point of the finer powder, the temperature being from about 5500° C. to about 17,000° C., to melt at least about 50% by weight of the finer powder to form essentially fine spherical particles of the melted portion, and rapidly and directly resolidifying the resulting high temperature treated material while the material is in flight, to form fine spherical particles having a particle size of less than about 20 micrometers in diameter. The particles are essentially free of elliptical shaped material and essentially free of elongated particles having rounded ends, and have an oxygen content of less than about 0.5% by weight, and a carbon content of no greater than the carbon content of the starting material.

This invention is related to application Ser. No. 07/161,535 entitled"Process For Producing Fine Copper Powder With Enhanced Sinterability"and Ser. No. 07/292,788 entitled "Process For Producing Fine CopperFlakes", both assigned to the same assignee as the present applicationand both filed concurrently herewith.

This invention relates to fine spherical copper powder particles and tothe process for producing the particles which involves mechanicallyreducing the size of an electrolytically produced dendritic copperstarting material by fluid energy or jet milling following by hightemperature processing to produce fine spherical particles having oxygencontents of less than about 0.5% by weight. More particularly the hightemperature process is a plasma process.

BACKGROUND OF THE INVENTION

Replacement of precious metal powder by copper powder for thick filmpaste hybrid circuit applications is a current trend in the electroniccircuit fabrication industry. Copper offers several economic andperformance advantages over precious metals. However, copper thick filmpaste technology is in its infancy, and many improvements still must bemade in the process. One of the improvement is reduced cost. One of theways by which costs may be reduced is to use a smaller diameter copperpowder which allows a thinner circuit trace to be fabricated.

One of the better commercial processes for producing such copperparticles is by gas atomization. Only a small percentage of the powderproduced by atomization is less than about 15 micrometers in maximumparticle size. An even smaller percentage is below about 3 micrometersin size. Therefore, yields are low and metal powder costs are high as aresult.

Another process for producing copper powders for thick film electroniccircuit applications is by chemical or electrical precipitation. Whilethese powders are made with a high yield under 10 micrometers indiameter, the particles are not smooth and spherical. This leads toproblems with flowability, packing density, and organic vehicle removal.

U.S. Pat. Nos. 3,909,241 and 3,974,245 to Cheney et al. relates to freeflowing powders which are produced by feeding agglomerates through ahigh temperature plasma reactor to cause at least partial melting of theparticles and collecting the particles in a cooling chamber containing aprotective gaseous atmosphere where the particles are solidified.

U.S. Pat. 4,264,354 to Cheetham relates to producing spherical dentalalloy powders by high frequency induction coil heating followed bycooling in a liquid medium.

In European Patent Application W08402864 published Aug. 2, 1984, thereis disclosed a process for making ultra-fine powder by directing astream of molten droplets at a repellent surface whereby the dropletsare broken up and repelled and thereafter solidified as describedthereinn. While there is a tendency for spherical particles to be formedafter rebounding, it is stated that the molten portion may formelliptical shaped or elongated particles with rounded ends.

U.S. Pat. Nos. 4,711,660 and 4,711,661 relate to spherical particles andprocess for producing same by mechanically reducing the particles sizeof the material and high temperature processing followed by rapidsolidification. The oxygen content of the spherical particles when thematerial is reduced in size by the preferred attritor milling istypically greater than about 0.8 % by weight. It is desirable that theoxygen content be lower than this value for better sintering and bettermechanical properties, etc.

SUMMARY OF THE INVENTION

In accordance with one aspect of this invention there is provided apowder material consisting essentially of fine spherical copperparticles which are essentially free of elliptical material andessentially free of elongated particles having rounded ends. Theparticles have a particle size of less than about 20 micrometers indiameter, have an oxygen content of less than about 0.5% by weight, anda carbon content of no greater than the carbon content of the startingmaterial.

In accordance with another aspect of this invention, there is provided aprocess for producing the above described particles which comprisesreducing the size of an electrolytically produced starting dendriticcopper powder material by fluid energy milling to produce a finerpowder, essentially all of which has a particle size of less than about20 micrometers in diameter, entraining the finer powder in a carrier gasand passing it through a high temperature zone at a temperature abovethe melting point of the finer powder, the temperature being from about5500° C. to about 17,000° C., to melt at least about 50% by weight ofthe finer powder to form essentially fine spherical particles of themelted portion, and rapidly and directly resolidifying the resultinghigh temperature treated material while the material is in flight.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an SEM photograph at 2000× of electrolytically produceddendritic copper powder that is the typical starting material for theprocess of the present invention.

FIG. 2 is an SEM photograph at 2000× of the above described powder afterjet milling according to the process of the present invention.

FIG. 3 is an SEM photograph at 2000× of the above processed powderswhich have been plasma processed according to the process of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above described drawings and description of some of the aspects ofthe invention.

The starting material of this invention is copper metal particles. Thematerial is dendritic copper powder. By dendritic is meantfern-or-tree-like single crystals feathery in appearance and produced byelectrolytic deposition. Typically the copper powder is electrolyticallyproduced because it is easier to reduce in size in the fluid energymilling step. The size of the material is of any size or diametertypically less than about 2,000 microns in diameter, measured at thewidest part of the particle.

The size of the starting material is first reduced to produce a finerpowder material. Essentially all of the material is reduced to aparticle size of less than about 20 micrometers in diameter as measuredby conventional techniques such as air or liquid settling, laserdiffractometry, or Microtrac. With Microtrac, it is assumed that theparticles are spherical, and therefore for non-spherical particles, thereported particle sizes are relative values.

The size reduction is accomplished by a group of processes commonlycalled "jet milling" or "fluid energy milling", including fluidized bedopposed jet milling, the "Coldstream" process in which a stream of gasand the starting material are impinged against a fixed target, etc. Allreferences made herein to "jet milling" or "fluid energy milling" areunderstood to refer to this group of processes. In the process of theinvention, there are no moving parts except for gas compressors toproduce the fluid energy stream. Energy is imparted to the particles bythe fluid or gas that is, by the velocity of the fluid. All of theseprocesses impart high velocities to the material which is being groundand impact the accelerated particles against each other or against asolid substrate at a sufficient force to shatter or break the particlesinto smaller fragments.

In U.S. Pat. Nos. 4,711,660 and 4,711,661 relate to particle sizereduction followed by high temperature processing and rapidsolidification to form spherical particles. These patents stressmedia/mechanical motion or vibration to reduce particle size. Thesepatents relate to processes in which the size reduction is done in aliquid medium and the material must be dried before subsequent hightemperature processing. Both of these steps increase the likelihood foroxidation of the powder. By contrast, according to this invention, thesize reduction can be done with the material in the dry state in aninert atmosphere. Only the correct size powder is produced and thereforethere is no need for screening or size classification before hightemperature processing. Furthermore, the processes of these patentsresult in powders which have a carbon content exceeding that required insome applications. This is due to the fact that size reduction occurstypically in a liquid organic medium which breaks down or is trappedwithin the powder particles. This results in an increase in the carboncontent of the powder. By contrast, the present invention is carried outwith the material in the dry state and the carbon content is thereforenot increased. Carbon content affects the resistivity and otherperformance measurements for copper in electronic applications. Whenfluid energy milling is used, the oxygen content in the resultantspherical powder particles is less than about 0.5% by weight, and thecarbon content is essentially no greater than that of the startingmaterial. Also the process operates at a higher efficiency than priorart methods of gas or water atomization or the processes of U.S. Pat.Nos. 4,711,660 and 4,711,661 because only the correct size powder isdischarged from the jet milling apparatus to convert it to sphericalparticles by high temperature processing. The prior art methods ofmechanical size reduction are batch processes. Therefore all materialundergoes high temperature processing, even if a portion of it is notthe correct size. Thus, more material must undergo the high temperatureprocessing to yield a given amount of product, and more post-hightemperature treatment classifying is necessary to yield the desiredfinal size distribution. The process of this invention yields a moreuniform size reduced material for subsequent high temperature processingthan does prior art processing. This is so because the fluid energymilling is a continuous process. The oversize powder is recycled to thefluid energy milling process while the correct size material which isfiner than the starting material is discharged from the mill forsubsequent high temperature process. This is important because meltingefficiency (the weight ratio of melted particles to total particles) isincreased when the material that is subjected to the high temperatureprocess is more uniform in size.

The preferred jet mill to accomplish size reduction is the fluidized bedopposed jet mill invented by Alpine. The mill is comprised of acylindrical grinding chamber with an Alpine classifier mounted at thetop. Compressed air, nitrogen, or inert gases is introduced into themill through three or more horizontally oriented nozzlescircumferentially spaced around the lower portion of the grindingchamber. Material is introduced into the chamber by a feeder at thebottom of the chamber or through a tube entering the grinding chamberabove the gas jets. Because of the gas flowing into the mill, thematerial which is being size reduced forms a fluidized bed at the bottomof the grinding chamber. Gas leaves the nozzles at supersonic velocitiesand accelerates the material to be reduced in size. Particles ofmaterial are entrained in each gas jet and impact near the center of thegrinding chamber with particles entrained in the other gas jets.Particles fracture and therefore, size reduction occurs at this stage ofthe process. The mixture of size reduced and unground material travelsupwards through the grinding chamber to the air classifer, which is afinned wheel (similar in appearance to a "squirrel cage" blower)rotating at a high speed (>5,000 rpm). The wheel rejects particles abovea certain size (which is adjustable) and returns these unground orpartially ground particles to the fluidized bed of the grinding chamber.The oversize material rejected by the classifier wheel is reentrained inthe gas jets for further grinding. Fine particles of the desired sizepass through the classifier wheel, where they are collected byconventional means, such as gas cyclones or filters. New startingmaterial is fed into the mill at a rate equal to the rate at which finesize reduced powder leaves the mill.

If a metal or metal alloy powder is size reduced by the above describedjet mill with nitrogen or an inert gas as the grinding/atmosphere gas,the oxygen content of the size reduced powder is only slightly greaterthan the starting oxygen content. No matter which gas is used formilling, contamination of the material other than by oxygen during sizereduction is minimal, even compared to other jet milling processes,because the material impacts and fractures against itself. Wear of thejet milling apparatus, which implies contamination of the material whichis being size reduced, is minimal. The above described eqipment offersmany advantages over conventional tumbling or stirred ball mills for thesize reduction of metal powders. In conventional mills, milling isusually conducted in an organic solvent, which leads to carboncontamination. This does not happen in the process of the presentinvention. Also, the size reduced material must be dried beforeconversion to essentially spherical particles, and oxidation is nearlyunavoidable.

The reduced size material is then entrained in a carrier gas such asargon and passed through a high temperature zone at a temperature abovethe melting point of the finer powder for a sufficient time to melt atleast about 50% by weight of the finer powder and form essentially fineparticles of the melted portion. Some additional particles can bepartially melted or melted on the surface and these can be sphericalparticles in addition to the melted portion. The preferred hightemperature zone is a plasma.

Details of the principles and operation of plasma reactors are wellknown. The plasma has a high temperature zone, but in cross section thetemperature can vary typically from about 5500° C. to about 17,000° C.The outer edges are at low temperatures and the inner part is at ahigher temperature. The retention time depends upon where the particlesentrained in the carrier gas are injected into the nozzle of the plasmagun. Thus, if the particles are injected into the outer edge, theretention time must be longer, and if they are injected into the innerportion, the retention time is shorter. The residence time in the plasmaflame can be controlled by choosing the point at which the particles areinjected into the plasma. Residence time in the plasma is a function ofthe physical properties of the plasma gas and the powder material itselffor a given set of plasma operating conditions and powder particles.Larger particles are more easily injected into the plasma while smallerparticles tend to remain at the outer edge of the plasma jet or aredeflected away from the plasma jet.

After the material passes through the plasma, it cools, and is rapidlyand directly solidified. Generally the major weight portion of thematerial is converted to spherical particles. Generally greater thanabout 75% and most typically greater than about 85% of the material isconverted to spherical particles by the high temperature treatment.Nearly 100% conversion to spherical particles can be attained. Theparticles are less than about 20 micrometers in diameter. The majorportion of the spherical particles are less than about 10 micrometers indiameter. The particle size of the plasma treated particles is largelydependent of the size of the material obtained in the size reductionstep. Most typically greater than about 99% of the particles are lessthan about 15 micrometers.

More preferred particle sizes are less than about 7 micrometers indiameter and most preferably less than about 3 micrometers in diameter,and it is preferred that the particles be greater than about 0.1micrometer in diameter.

After cooling and subsequent resolidification, the resulting hightemperature treated material can be classified to remove the majorspheroidized particle portion from the essentially non-spheroidizedminor portion of particles and to obtain the desired particle sizedistribution. The classification can be done by standard techniques suchas screening or air classification.

The unmelted minor portion can then be reprocessed according to theinvention to convert it to fine spherical particles.

The powder materials of this invention are essentially smooth-surfacedspherical particles which are essentially free of elliptical shapedmaterial and essentially free of elongated particles having roundedends. These characteristics can be present in the particles made by theprocess described in European Patent Application W08402864 as previouslymentioned.

Furthermore, the levels of chemical contamination (carbon, oxygen, etc.)in the final product of this invention are much lower than those foundin the spherical particles made by prior art high temperature processes.The oxygen levels in the particles produced by the process of thepresent invention are typically less than about 0.5% by weight and moretypically less than about 0.3% by weight with levels as low as about0.15% by weight can be achieved.

Spherical particles have an advantage over non-spherical particles inthick film electronic circuit applications. The lower surface area ofspherical particles as opposed to non-spherical particles of comparablesize, and the flowability of spherical particles makes sphericalparticles easier to mix with binders and easier to remove binders from.

To more fully illustrate this invention, the following non-limitingexample is presented.

EXAMPLE

Dendritic morphology copper powder produced by electrolytic depositionis reduced in size by opposed jet milling using compressed air at a jetinlet pressure of from about 82 to about 85 pounds per square inch. Thereduced size powder is then plasma melted using argon as the plasma gasand 10.5 kilowatts (300 amperes, 35 volts) of input powder the plasmatorch. Scanning electron microscope pictures at a magnification of 2000×are shown in the Figures: the starting dendritic copper powder in FIG.1, the powder reduced in size by jet milling in FIG. 2, and the plasmamelted powder before classification in FIG. 3. Note in FIG. 3 there aresubmicron particles clinging to the fine spherical copper particles.These can be easily removed by air classification. The particle size asmeasured by Microtrac and the analyses of the powder at various stagesin the process are shown in the Table below.

                  TABLE                                                           ______________________________________                                                                        plasma                                                     Starting powder                                                                         jet milled                                                                             melted                                        ______________________________________                                        Mean size, micrometers                                                                       12.4         5.5      7.6*                                     Std deviation, micrometers                                                                    6.8         3.3      5.5*                                     wt. % <7 micrometers                                                                         21.9        76.0     55.0*                                     wt. % <3 micrometers                                                                          3.6        19.5     14.5*                                     carbon, wt ppm  380         340      68                                       Oxygen, wt ppm 1100        1100     1300                                      Nitrogen wt. ppm                                                                             <10          17      <10                                       ______________________________________                                         *The apparent size increase during plasma melting is due to the small         sample size and to inedaquate collection of the <3 micron fraction of the     powder. The size remains essentially the same after plasma melting.      

Note that the carbon and oxygen contents are essentially the same beforeand after plasma melting.

While there has been shown and described what are at present consideredthe preferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims.

What is claimed is:
 1. A process for producing fine spherical copperpowder particles, said process comprising:(a) reducing the size of aelectrolytically produced starting dendritic copper powder material byfluid energy milling to produce a finer powder, essentially all of whichhas a particle size of less than about 20 micrometers in diameter; (b)entraining said finer powder in a carrier gas and passing said powderthrough a high temperature zone at a temperature above the melting pointof said finer powder said temperature being from about 5500° C. to about17,000° C., to melt at least about 50% by weight of said finer powder toform essentially fine spherical particles of said melted portion; and(c) rapidly and directly resolidifying the resulting high temperaturetreated material while said material is in flight, to form finespherical particles having a particle size of less than about 20micrometers in diameter, said particles being essentially free ofelliptical shaped material and essentially free of elongated particleshaving rounded ends, said particles having an oxygen content of lessthan about 0.5% by weight, and a carbon content of no greater than thecarbon content of said starting material.
 2. A process of claim 1wherein the size of said starting material is reduced by fluidized bedopposed jet milling said material to produce said finer powder.
 3. Aprocess of claim 2 wherein the size of said starting material is reducedby fluidized bed opposed jet milling.
 4. A process of claim 1 whereinafter said resolidification, said high temperature treated material isclassified to obtain the desired particle size of said sphericalparticles.
 5. A process of claim 1 wherein said high temperature zone isa plasma.
 6. A powder material consisting essentially of sphericalcopper particles, said powder material being essentially free ofelliptical shaped material and essentially free of elongated particleshaving rounded ends, said powder material having a particle size of lessthan about 20 micrometers in diameter, said powder material being madeby jet milling a starting electrolytically produced material which isdendritic copper powder in the dry state with an inert gas by impactingthe particles of said starting material against each other to fracturesaid particles of said starting material, followed by high temperatureprocessing and direct solidification of the resulting high temperaturetreated material, said powder material having an oxygen content of lessthan about 0.5% by weight and a carbon content of no greater than thecarbon content of said starting material.
 7. A powder material of claim6 wherein the maximum particle size of said spherical particles is lessthan about 10 micrometers in diameter.
 8. A powder material of claim 6wherein said maximum particle size is greater than about 0.1 micrometerin diameter.
 9. A powder material of claim 7 wherein the particle sizeis greater than about 0.1 micrometer in diameter.